1. Field of the Invention
The present invention relates generally to the production of structural ceramic materials, and more particularly to a process for producing reinforced aluminum oxide ceramic materials by application of a liquid aluminum oxide precursor onto a reinforcement fabric and pyrolytic conversion of the precursor to an aluminum oxide ceramic matrix phase.
Structural ceramic materials are utilized in a wide variety of high temperature and high strength applications. Such ceramic materials usually comprise a composite including reinforcement phase, usually a fabric formed from a plurality of individual reinforcement fibers, and a matrix phase which impregnates the reinforcement fabric. The reinforcement fabric provides the structural component of the composite, enhancing the tensile and flexural strength. The matrix phase bonds the filaments, yarns, and layers of fabric and enhances the bulk and compressive strength of the composite article.
Because of its low dielectric constant, it would be desirable to utilize an aluminum oxide ceramic as the matrix phase for certain ceramic composite articles. Heretofore, however, the impregnation of a reinforcement phase with aluminum oxide has been problematic. While powdered aluminum oxide can be introduced and converted to a semi-continuous phase by sintering, the high temperatures and pressures required will usually degrade any reinforcement materials which may be present in the ceramic. Moreover, penetration of powders within the reinforcement fabric is uneven resulting in a non-uniform distribution of the matrix phase in the composite.
It is therefore an object of the present invention to provide methods for preparing aluminum oxide ceramic composites which have an even distribution of the ceramic within the reinforcement fabric and which do not require high temperature processing. It would be particularly desirable to provide a liquid aluminum oxide precursor substance which can be uniformly introduced to the reinforcement fabric and converted to the ceramic matrix phase by a relatively low temperature conversion.
2. Description of the Background Art
The preparation of ceramic materials from organic precursors is described in Eror and Anderson, "Polymeric Precursor Synthesis of Ceramic Materials", Better Ceramics Through Chemistry II, C. J. Brinkes, D. E. Clark, D. R. Ulrich, eds. Material Research Society, Pittsburg, Pa, pp. 571-577 (1986). Polybasic acid chelates are formed with certain cations and undergo polyesterification when heated in a polyhydroxyl alcohol. The resulting polymeric glass has the cations uniformly distributed throughout and may be calcined at low temperature to yield fine particulate oxides. U.S. Pat. No. 3,330,697 to Pechini describes a process for preparing titanium, zirconium, and niobium ceramics as capacitor dielectrics. The ceramics are prepared by dissolving an appropriate salt in a polyhydroxyl alcohol in the presence of a polybasic acid to form a chelate. The addition of ethylene glycol causes esterification and polymerization occurs upon heating. The resulting liquid can be calcined to form the ceramic. Copending applications Ser. No. 878,454 and Ser. No. 046,956 describe the preparation of ceramic composites by introducing a polycarbosilane onto a fiber reinforcement fabric, curing the polycarbosilane at an elevated temperature under non-oxidizing conditions, and converting the cured polycarbosilane to a silicon carbide ceramic by pyrolysis.